1. Field of the Invention
This invention relates to pneumatic tires, and more particularly to a pneumatic studless tire for all seasons having considerably improved traction performance, braking performance and steering performance in running on snow and ice road surfaces (hereinafter referred to as ice-snow performances simply) without degrading the steering stability, durability and wear resistance required in summer season.
2. Description of the Related Art
Recently, it is increasing to demand so-called all season tires capable of using in winter season likewise summer season without tire change. Even in the winter season, the tire of this type has the same dry gripping property, wet gripping property, steering property, durability and low fuel-consumption as in the summer season and further possesses sufficient traction and braking performances on ice and snow roads.
As a tread rubber used in the tire of this type, it is required to reduce a hardness at a low temperature in a tread rubber for summer season. In this connection, there is known a method of using a polymer having a low glass transition point, or a method of using a softening agent to properly maintain a modulus of elasticity at low temperature.
However, the former method develops substantially the performance on ice-snow temperature region owing to hysteresis property of the polymer, but has a problem that the braking performance and steering property on wet road surface and dry road surface are insufficient. The latter method is disclosed in JP-A-55-135149, JP-A-58-199203, JP-A-60-137945 and the like and has a problem that the use of softening agent badly affects the wear resistance and durability in the running on general-purpose road in comparison with the improvement of ice-snow performances.
In the aforementioned conventional techniques, the ice-snow performances at a relatively lower temperature region of not higher than -5.degree. C. or so-called dry-on-ice state are certainly improved, but sufficient friction coefficient is not obtained in the ice-snow performances at a wet state near to 0.degree. C. or so-called wet-on-ice state, so that it can not be said that the traction performance, braking performance and steering stability are sufficiently improved.